The present invention relates to an air conditioner which is capable of cooling and heating a room, as well as defrosting at a room heating time.
An air conditioner of the above type generally has a compressor, a four-way valve, an outdoor side heat exchanger, an expansion valve which serves as a pressure reducing device, and an indoor side heat exchanger; which components are sequentially connected via a coolant tube, thereby constructing a refrigeration cycle circuit which is capable of performing cooling (defrosting) and heating operations.
Abnormally high pressure might occur in the air conditioner at the high pressure side of the refrigeration cycle, during the heating operation, due to a variation in the atmospheric temperature. Thus, one conventional type of air conditioner, such as that disclosed in Japanese Utility Publication No. 1076/1978, has a releasing circuit for releasing the generated high pressure to the low pressure side of the refrigeration cycle circuit, and a switching control valve which is provided within the releasing circuit.
When a cooling (defrosting) operation is carried out, the four-way valve is switched to the room cooling side, and the compressor is then driven. Thus, coolant sequentially flows from the compressor, through the four-way valve, outdoor side heat exchanger, expansion valve and indoor side heat exchanger, thereby forming a cooling cycle (defrosting cycle).
When a heating operation is performed, the four-way valve is switched to a room heating side, and the coolant sequentially flows from the compressor, through the four-way valve, indoor side heat exchanger, expansion valve and outdoor side heat exchanger, thereby forming a heating cycle.
When abnormal pressure is produced at the high pressure side of the refrigeration cycle, due to an overload at the heating operation, the control valve is opened, with the result that the high pressure is released to the low pressure side of the refrigeration cycle.
In the refrigeration cycle circuit of the above-described type, the throttling amount of the expansion valve is normally controlled on the basis of the refrigeration cycle temperature detected by a sensor such as a heat sensitive tube provided at the suction side of the compressor. However, when the defrosting operation is carried out by a reverse cycle (cooling cycle) during the heating operation, i.e., when the heating operation is temporarily transferred to the cooling operation to remove frost adhered to the outdoor side heat exchanger, extremely cooled coolant flows to the suction side of the compressor. Therefore, this low temperature is detected by the sensor and, since the defrosting operation is performed in a state wherein the throttling amount of the expansion valve is increased, the expansion valve is substantially closed. Consequently, the coolant hardly flows in the refrigeration cycle, during the defrosting operation, resulting in a deterioration of the defrosting performance.
In order to solve the above-described problem, it is considered that a shortcircuit between the outdoor side heat exchanger and the indoor side heat exchanger is created to inhibit the refrigerant from passing through the expansion valve at the defrosting time. However, in this case, a switching control valve for the shortcircuit is additionally needed by the control valve of the above-described releasing circuit. Since the control valve is expensive, it is not economically feasible to employ two control valves. Further, another control means for controlling these two control valves is also required, thereby so increasing the manufacturing cost as not to be practical.